The present invention relates generally to a process for preparing sintered aluminum nitride bodies and to bodies resulting from the process. The present invention relates more particularly to a process for preparing sintered aluminum nitride bodies having a thermal conductivity as high as about 285 watts/meter.multidot..degree.K (W/m-K).
Aluminum nitride (AlN) is subject to increasing interest as a microelectronic substrate material. With a thermal conductivity approaching that of berylia (BeO) and a thermal expansion coefficient well matched to silicon, AlN represents an attractive alternative in high power or multi-chip module applications.
At room temperature, single crystal AlN has a theoretical thermal conductivity of 319 W/m-K. Polycrystalline ceramics tend to have lower thermal conductivities than single crystal AlN due to a number of factors. The factors include random orientation of AlN grains, crystalline lattice impurity levels and existence of crystalline grain boundary phases with even lower thermal conductivities.
F. Miyashiro et al., in "High Thermal Conductivity Aluminum Nitride Ceramic Substrates and Packages", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol 13, No 2, 313-19 (June 1990), suggest that three key technologies are very important if one is to realize the highest thermal conductivity by sintering. The technologies are: reducing or minimizing oxygen content of AlN powders; proper choice and quantity of additives; and sintering conditions in terms of temperature, time and atmosphere. They show, in FIGS. 8-10: sintering at 1800.degree. Centigrade (.degree.C.) and 1900.degree. C. for two hours with three weight percent (wt-%) yttria (Y.sub.2 O.sub.3); sintering at 1800.degree. C. in the presence of varying amounts of Y.sub.2 O.sub.3 ; and sintering at 1800.degree. C. for 24 hours with three wt-% Y.sub.2 O.sub.3 with and without a reductive atmosphere. They suggest, in FIGS. 10 and 11, that a reducing atmosphere yields the highest thermal conductivity
H. Buhr et al., in "Phase Composition, Oxygen Content, and Thermal Conductivity of AlN(Y.sub.2 O.sub.3) Ceramics", J. Am. Ceram. Soc. 74[4], 718-723 (1991), disclose sintering cold isostatically pressed cylindrical compacts under a pressure of 0.2 MPa nitrogen. They employ heating rates of 16 to 30K per minute, sintering temperatures of 1750.degree. to 1790.degree. C., and sintering times of between one and three hours.
A. V. Virkar et al., in "Thermodynamic and Kinetic Effects of Oxygen Removal on the Thermal Conductivity of Aluminum Nitride", J. Am. Ceram. Soc. 72[11], 2031-42 (1989), fabricate polycrystaline AlN ceramics with various rare earth oxides and alkaline earth oxides. They sinter/anneal samples of the ceramics at 1850.degree. C. for up to 1000 minutes. They obtain thermal conductivities as high as 200 W/m-K.
K. Watari et al., in "Sintering Chemical Reactions to Increase Thermal Conductivity of Aluminum Nitride", J. Mater. Sci. 26, 4727-32 (1991), discuss chemical reactions to increase thermal conductivity by decreasing oxygen contents during AlN sintering with an Y.sub.2 O.sub.3 additive in a reducing nitrogen atmosphere with carbon. They wrap cold isostatic pressed bodies formed from admixtures of AlN powder and Y.sub.2 O.sub.3 powder and sinter the wrapped bodies at temperatures of 1773, 1873, 1973, 2073 and 2173K for one hour in a 0.1 MPa nitrogen gas atmosphere. They also sinter at 2173K in the same atmosphere for two, three and five hours. They use a heating rate of 15K/minute and report thermal conductivities as high as 220 W/m-K.
T. A. Guiton et al., in "Optimization of Aluminum Nitride Thermal Conductivity Via Controlled Powder Processing", Mat. Res. Soc. Symp. Proc., Vol 271, 851-56 (1992), suggest that thermal conductivity is strongly dependent on oxygen chemistry and sintering parameters. They disclose two sets of sintering parameters, denominated as "Cycle 1" and "Cycle 2" in Table II (page 852). Cycle 2 includes a heating rate of 2.5.degree. C./min, a sintering temperature of 1850.degree. C., a sintering time of 3 hours, a cooling rate of 1.degree. C./min and a cooling temperature of 1500.degree. C.
Y. Kurokawa et al., in "Highly Thermal Conductive Aluminum Nitride Substrates", ISHM Proceedings, 654-61 (1987), report thermal conductivity measurements of 160 to 260 W/m-K for AlN substrates. They prepare substrates by firing an admixture of AlN powder and a CaC.sub.2 powder reductant at 1900.degree. C.
U.S. Pat. No. 4,847,221 discloses a process for preparing sintered AlN bodies as well as the resultant bodies. The process comprises firing an admixture of AlN powder and one or more rare earth compounds in an amount of 0.01 to 15 wt-% in a reducing atmosphere at a temperature of 1550.degree. C. to 2050.degree. C. for four hours or more. The reducing atmosphere preferably contains at least one of CO gas, H.sub.2 gas, and C (gaseous or solid phase). The resultant bodies have thermal conductivities as high as 272 W/m-K.
U.S. Pat. No. 4,778,778 reports, at column 2, lines 10-26, a particular sintering cycle that is described in a copending application. The cycle provides high thermal conductivities without using very high purity aluminum nitride powder. The cycle includes: increasing the temperature of a compacted AlN body from room temperature to a sintering temperature at a rate of no more than 250.degree. C. per hour (.degree.C./hr); sintering the body in an inert atmosphere at a temperature of 1600.degree. C. to 1900.degree. C.; and cooling the sintered body at a rate of no more than 300.degree. C./hr. The '778 patent discloses an improvement upon this cycle. The improvement includes introducing an amount of hydrogen gas along with the inert gas up to a temperature of 1200.degree. C., after which pure inert gas is introduced. The heating rate is 10.degree. C./hr to 200.degree. C./hr, preferably 20.degree. C./hr to 80.degree. C./hr. The cooling rate is preferably between 100.degree. C./hr and 275 .degree. C./hr. The sintering cycle used in the example is as follows: 25.degree. C./hr to 800.degree. C., 33.degree. C./hr to 1000.degree. C., 80.degree. C./hr to 1500.degree. C., 300.degree. C./hr to 1800.degree. C., soak for six hours, and cooldown at 140.degree. C./hr.